The Buddha gives this instruction in the Griha Vinaya (Rules for Householders, Dharmika Sutra, Kshudraka Agama):

Let him not destroy, or cause to be destroyed,
any life at all, or sanction the acts of those who do so.
Let him refrain even from hurting any creature,
both those that are strong and those that tremble in the world.

If we fail to understand the universality of this injunction, the Buddha clarifies (Kshudraka Agama):

Whether they be creatures of the land or air,
whoever harms here any living being,
who has no compassion for all that live,
let such a one be known as depraved.

And in the Anguttara Agama:

I am a friend of the footless,
I am a friend of all bipeds,
I am a friend of those with four feet,
I am a friend of the many-footed.
…
May all creatures, all breathing things,
all beings one and all, without exception,
experience good fortune only.
May they not fall into any harm.

Should we intend to skirt the First Precept by claiming innocence of the deed if others do the killing for us, He adds (Kshudraka Agama):

One should not kill any living being,
nor cause it to be killed,
nor should one incite any other to kill.
Do never injure any being,
whether strong or weak, in this entire universe!

A Buddhist monk who studied and meditated on the teachings of Buddha for over a decade wanted to speak with an enlightened one who dwelled alone in a small temple high in the mountains. He prepared for his journey eager to hear his teaching. After 7 days of rigorous hiking he reached the temple.

The master greeted him and welcomed him inside. The master put some tea on to boil and they sat in silence. The monk became very eager and unsettled in not hearing his teaching. The master poured the tea and sat with the monk. The monk didn’t drink his tea and began questioning his master of Buddha.

The master replied.. “You have studied Buddhism for 20 years and can not simply enjoy this cup of tea?” The monk wept…. as another step to enlightenment lay under his feet….

Consciousness (mind and mentation) is “the stuff” what we mistake to be subject and object, me and mine, us and things. There is no substantial me, mine, us, things, but only internal representations in our experience.

When you see a movie, the actors and the scenery are not there in the theater. They are images on film, projected on a screen. These images look like various persons and things, but they are all “made” of the same stuff: light and shadow.

In the same way, our every experience is only in and of consciousness, taking the form of (transforming into) subject and object, self and beings, phenomena and their characteristics. Our experience is never extra-mental. It is always internal.